1. Field of the Invention
The present invention relates to an ink jet recording head for performing recording by discharging an ink droplet from a discharge port and by adhering the ink droplet onto a recording medium.
2. Related Background Art
As one of the ink discharging methods in ink jet recording apparatuses, which are now used widely, there is a method utilizing an electro-thermal converting element (heater). The principle is that heat is generated by applying an electrical signal to the electro-thermal converting element disposed in a pressure chamber to which ink is supplied, thereby heating the ink near the electro-thermal converting element instantaneously to boil the ink, with the result that the ink is discharged from a discharge port externally by great bubble pressure abruptly generated due to phase change. An ink jet recording head of this type has advantages that the structure is simple and that integration of ink flow paths is facilitated.
In such an ink jet recording head, there is a case where recording is performed by forming an ink droplet finer than the normal ink droplet in order to realize highly fine recording. To this end, there has been proposed an arrangement in which the discharging of the larger ink droplet and the discharging of the smaller ink droplet are used properly. In general, it can be considered that the discharge port and the electro-thermal converting element must be miniaturized in order to discharge the smaller ink droplet.
Concretely, in order to reduce the size of the discharged liquid droplet, the discharge port area is made smaller substantially in inverse proportion to the discharge amount. For example, when an ink droplet of 5 pl is preferably discharged from a discharge port having a diameter of 16 to 16.5 xcexcm (area is 201 to 214 xcexcm2), it is considered to be preferable that a discharge port for discharging a smaller ink droplet (for example, 4 pl) has a diameter of about 15.5 xcexcm (area is 189 xcexcm2) and a discharge port for discharging a more smaller ink droplet (for example, 2 pl) has a diameter of about 10.5 xcexcm (area is 87 xcexcm2).
According to a normal design method, when the discharge port and the electro-thermal converting element are miniaturized in order to discharge the small ink droplet, the pressure chamber within which the electro-thermal converting element is installed is also miniaturized accordingly. An ink flow path for connecting the pressure chamber to a common liquid chamber is designed to have a width the same as the width of the pressure chamber. That is to say, in correspondence to the miniaturization of the ink droplet, the discharge port, electro-thermal converting element and pressure chamber are all miniaturized at the same rate, and the pressure chamber and the ink flow path are formed to have the same width.
However, in such a design method, it was found that there is a case where the minute ink droplet may not be discharged successfully. That is to say, even if a small liquid discharging nozzle is constructed by reducing the dimensions of the discharge port, electro-thermal converting element and pressure chamber which can discharge the normal ink droplet (large ink droplet) successfully in proportion to the reduction of the ink amount of the ink droplet to be discharged, in many cases, good ink droplet discharging cannot be achieved. It is guessed that one of factors causing the poor discharging is the fact that flow resistance is increased by the miniaturization of the discharge port.
Explaining this more concretely, the viscosity resistance of the discharge port is increased in inverse proportion to fourth power of the area of the discharge port. That is to say, when the discharge port is miniaturized in correspondence to the miniaturization of the ink droplet, since the viscosity resistance is increased, in order to maintain the proper discharging condition if the viscosity resistance is increased, the bubbling power generated by the electro-thermal converting element must be increased. In the above-mentioned conventional design method, although it was considered that the bubbling power of the electro-thermal converting element can merely be decreased in accordance with the miniaturization of the discharged ink droplet, actually, it is considered that, in addition to this, bubbling power required for is overcoming the increased viscosity resistance should be considered. Accordingly, the minimum bubbling power required for discharging the ink droplet from the discharge port successfully cannot eventually be reduced much in comparison with the case where the large ink droplet is discharged, because the fact that the power can be reduced in accordance with the miniaturization of the ink droplet to be discharged is cancelled out by the fact that the power must be increased to cope with the increase in viscosity resistance, with the result that the size of the electro-thermal converting element cannot be reduced much.
Further, due to limitation of the design of the ink jet recording head, in a certain case, the distance between the electro-thermal converting element and the discharge port cannot be shortened in accordance with the miniaturization of the ink droplet to be discharged and the discharge port. That is to say, there is a case where the distance between the electro-thermal converting element and the discharge port becomes constant by forming the discharge port for discharging the large ink droplet and the discharge port for discharging the small ink droplet in a single substrate and installing the corresponding electro-thermal converting elements in parallel on the single substrate in order to simplify the construction and the manufacturing process. In this case, even when the diameter of the discharge port is decreased in accordance with the miniaturization of the ink droplet to be discharged, the distance to the discharge port cannot be shortened, thereby causing bad balance. Since the distance to the discharge port is long relatively, the energy required for discharging the ink out of the discharge port becomes relatively great.
Also for this reason, the minimum energy required for discharging the ink droplet cannot be reduced much in comparison with the rate of reduction of the amount of the ink droplet and the rate of the miniaturization of the discharge port, and the size of the electro-thermal converting element cannot be reduced much in comparison with the electro-thermal converting element for discharging the large ink droplet.
For example, in the above-mentioned example, if the electro-thermal converting element used for discharging the ink droplet of 5 pl has a square shape of 26 xcexcmxc3x9726 xcexcm (or two elements having a dimension of 12.5 xcexcmxc3x9728 xcexcm), the electro-thermal converting element for discharging the ink droplet of 4 pl is required to have a square shape of about 24 xcexcmxc3x9724 xcexcm, and, the electro-thermal converting element required for discharging the ink droplet of 2 pl becomes a square shape of about 22 xcexcmxc3x9722 xcexcm (or two elements having a dimension of about 11.5 xcexcmxc3x9727 xcexcm). As such, while the discharge port can be miniaturized in accordance with the reduction of the dimensions of the ink droplet, in comparison with this, the electro-thermal converting element cannot be miniaturized so much.
Further, the pressure chamber for discharging the small ink droplet cannot be miniaturized so much since it must contain the electro-thermal converting element. When a margin of 2 xcexcm is provided around an outer periphery of the electro-thermal converting element in consideration of alignment error of a flow path forming member, for example, the pressure chamber required for discharging the ink droplet of 5 pl must have a square shape of (26+4) xcexcmxc3x97(26+4) xcexcm=30 xcexcmxc3x9730 xcexcm (bottom area is 900 xcexcm2) or a square shape of (12.5xc3x972+3+4) xcexcmxc3x97(28+4) xcexcm=32 xcexcmxc3x9732 xcexcm (bottom area is 1,024 xcexcm2). In contrast, the pressure chamber required for discharging the ink droplet of 4 pl has a square shape of (24+4) xcexcmxc3x97(24+4) xcexcm=28 xcexcmxc3x9728 xcexcm (bottom area is 784 xcexcm2), and the pressure chamber required for discharging the ink droplet of 2 pl has a square shape of (22+4) xcexcmxc3x97(22+4) xcexcm=26 xcexcmxc3x9726 xcexcm (bottom area is 676 xcexcm2) or a rectangular shape of (11.5xc3x972+3+4) xcexcmxc3x97(27+4) xcexcm=30 xcexcmxc3x9731 xcexcm (bottom area is 930 xcexcm2).
As such, when the minute ink droplet is discharged, the electro-thermal converting element and the pressure chamber cannot be miniaturized so much in comparison with the rate of the miniaturization of the discharge port.
As mentioned above, since an ink flow path having the same width of that of the pressure chamber is normally provided, when the pressure chamber is not miniaturized so much, the width of the ink flow path is not reduced so much. As a result, of the bubbling power of the electro-thermal converting eminent, a power component directed toward the ink flow path side rather than the discharge port side and not contributing to the discharging of the ink droplet is increased so as to cause great loss, thereby worsening the energy efficiency.
Accordingly, an object of the present invention is to provide an ink jet recording head in which loss can be reduced and energy efficiency can be enhanced also in a nozzle for discharging a small ink droplet, on the basis of a unique designing method, which is unknown in the prior art.
The present invention provides an ink jet recording head in which pressure chambers are connected to a plurality of respective ink flow paths branched from a common liquid chamber, discharge ports are communicated with the respective pressure chambers, ink supplied from the common liquid chamber to each pressure chamber can be discharged from the corresponding discharge port by pressure generated in the pressure chamber by heat from a corresponding electro-thermal converting element, and wherein the plurality of pressure chambers include a small liquid droplet pressure chamber for discharging a small liquid droplet and a large liquid droplet pressure chamber for discharging a large liquid droplet, and, regarding the ink flow path for the small liquid droplet connected to the small liquid droplet pressure chamber, the small liquid droplet pressure chamber, the ink flow path for the large liquid droplet connected to the large liquid droplet pressure chamber and the large liquid droplet pressure chamber, when sections substantially perpendicular to ink flows directed from the respective ink flow paths to the respective pressure chambers are looked at, a relationship between a sectional area SS of the small liquid droplet ink flow path, a sectional area SRS of the small liquid droplet pressure chamber, a sectional area SL of the large liquid droplet ink flow path and a sectional area SRL of the large liquid droplet pressure chamber satisfies SS/SRS less than SL/SRL. Further, it is preferable that a relationship between the sectional area SRS of the small liquid droplet pressure chamber and the sectional area SRL of the large liquid droplet pressure chamber and an ink amount IS of the small liquid droplet discharged from the small liquid droplet pressure chamber and an ink amount IL of the large liquid droplet discharged from the large liquid droplet pressure chamber satisfies SRS/SRL greater than IS/IL.
Further, it is preferable that a relationship between a volume VRS of the small liquid droplet pressure chamber and a volume VRL of the large liquid droplet pressure chamber and the ink amount IS of the small liquid droplet discharged from the small liquid droplet pressure chamber and the ink amount IL of the large liquid droplet discharged from the large liquid droplet pressure chamber satisfies VRS/VRL greater than IS/IL.
Further, SL=SRL and SS less than SRS may be satisfied.
Further, it is preferable that the following relationships are satisfied:
SLbxe2x89xa6SSb less than 1.93 SLb
SLb=RLf/(RLf+RLb)xc3x97SLe
SSb=RSf/(RSf+RSb)xc3x97SSe
where
SLb: flow resistance of large liquid droplet side;
SSb: flow resistance of small liquid droplet side;
RLf: flow resistance from electro-thermal converting element of large liquid droplet pressure chamber to corresponding discharge port;
RLb: flow resistance from electro-thermal converting element of large liquid droplet ink flow path to common liquid chamber;
SLe: effective bubbling area of the large liquid droplet electro-thermal converting element;
RSf: flow resistance from electro-thermal converting element of small liquid droplet pressure chamber to corresponding discharge port;
RSb: flow resistance from electro-thermal converting element of small liquid droplet ink flow path to common liquid chamber; and
SSe: effective bubbling area of small liquid droplet electro-thermal converting element.
Further, the following relationships or equations may be satisfied:   Rf  =      n    ⁢                  ∫        0        H            ⁢                        D          ⁡                      (            x            )                          ⁢                  xe2x80x83                ⁢                  ⅆ          x                ⁢                  /                ⁢                              S            ⁡                          (              x              )                                2                    xe2x80x83D(x)=12.0xc3x97(0.33+1.02xc3x97(a(x)/b(x)+b(x)/a(x)))
where
Rf: flow resistance from electro-thermal converting element to corresponding discharge port;
H: distance from electro-thermal converting element to corresponding discharge port;
x: distance from electro-thermal converting element;
S(x): sectional area of ink flow path at position of distance x;
D(x): section coefficient of ink flow path at position of distance x;
a(x): height of ink flow path at position of distance x;
b(x): width of ink flow path at position of distance x; and
xcex7: ink viscosity, and,   Rb  =      n    ⁢                  ∫        0        L            ⁢                        D          ⁡                      (            y            )                          ⁢                  xe2x80x83                ⁢                  ⅆ          y                ⁢                  /                ⁢                              S            ⁡                          (              y              )                                2                    xe2x80x83D(y)=12.0xc3x97(0.33+1.02xc3x97(c(y)/d(y)+d(y)/c(y)))
where
Rb: flow resistance from electro-thermal converting element to common liquid chamber;
L: distance from center of electro-thermal converting element to common liquid chamber;
y: distance from the common liquid chamber;
S(y): sectional area of ink flow path at position of distance y;
D(y): section coefficient of ink flow path at position of distance y;
c(y): height of ink flow path at position of distance y; and
d(y): width of ink flow path at position of distance
Further, the following relationships may be satisfied:   Rf  =      n    ⁢                  ∑                  n          =          1                k            ⁢                        D          ⁡                      (                          x              n                        )                          ⁢                  (                                    x              n                        -                          x                              n                -                1                                              )                ⁢                  /                ⁢                              S            ⁡                          (                              x                n                            )                                2                    xe2x80x83D(xn)=12.0xc3x97(0.33+1.02xc3x97(a(xn)/b(xn)+b(xn)/a(xn)))
where
Rf: flow resistance from electro-thermal converting element to corresponding discharge port;
k: division number of distance from electro-thermal converting element to corresponding discharge port;
xn: distance from electro-thermal converting element to n-th division position when distance from electro-thermal converting element to corresponding discharge port is divided into k sections;
S(xn) sectional area of ink flow path at position of xn;
D(xn): section coefficient of ink flow path at position of xn;
a(xn): height of ink flow path at position of xn;
b(xn): width of ink flow path at position of xn; and
xcex7: ink viscosity, and,   Rb  =      n    ⁢                  ∑                  n          =          1                l            ⁢                        D          ⁡                      (                          y              n                        )                          ⁢                  (                                    y              n                        -                          y                              n                -                1                                              )                ⁢                  /                ⁢                              S            ⁡                          (                              y                n                            )                                2                    xe2x80x83D(yn)=12.0xc3x97(0.33+1.02xc3x97(c(yn)/d(yn)+d(yn)/c(yn)))
where
Rb: flow resistance from electro-thermal converting element to common liquid chamber;
l: division number of distance from center of electro-thermal converting element to common liquid chamber;
yn: distance from common liquid chamber to n-th division position when distance from center of electro-thermal converting element to common liquid chamber is divided into l sections;
S(yn): sectional area of ink flow path at position of yn;
D(yn): section coefficient of ink flow path at position of yn;
c(yn): height of ink flow path at position of yn; and
d(yn): width of ink flow path at position of yn.
Further, the following relationships may be satisfied:   Rf  =      ρ    ⁢                  ∫        0        H            ⁢                        ⅆ          x                ⁢                  /                ⁢                  S          ⁡                      (            x            )                              
where
Rf: flow resistance from electro-thermal converting element to corresponding discharge port;
H: distance from electro-thermal converting element to corresponding discharge port;
x: distance from electro-thermal converting element;
S(x): sectional area of ink flow path at position of distance x; and
xcfx81: ink density, and,   Rb  =      ρ    ⁢                  ∫        0        L            ⁢                        ⅆ          y                ⁢                  /                ⁢                  S          ⁡                      (            y            )                              
where
Rb: flow resistance from electro-thermal converting element to common liquid chamber;
L: distance from center of electro-thermal converting element to common liquid chamber;
y: distance from the common liquid chamber; and
S(y): sectional area of ink flow path at position of distance y.
Further, the following relationships may be satisfied:   Rf  =      ρ    ⁢                  ∑                  n          =          1                k            ⁢                        (                                    x              n                        -                          x                              n                -                1                                              )                ⁢                  /                ⁢                  S          ⁡                      (                          x              n                        )                              
where
Rf: flow resistance from electro-thermal converting element to corresponding discharge port;
k: division number of distance from electro-thermal converting element to corresponding discharge port;
xn: distance from electro-thermal converting element to n-th division position when distance from electro-thermal converting element to corresponding discharge port is divided into k sections;
S(xn): sectional area of ink flow path at position of xn; and
xcex7: ink viscosity, and,   Rb  =      ρ    ⁢                  ∑                  n          =          1                l            ⁢                        (                                    y              n                        -                          y                              n                -                1                                              )                ⁢                  /                ⁢                  S          ⁡                      (                          y              n                        )                              
where
Rb: flow resistance from electro-thermal converting element to common liquid chamber;
l: division number of distance from center of electro-thermal converting element to common liquid chamber;
yn: distance from common liquid chamber to n-th division position when distance from center of electro-thermal converting element to common liquid chamber is divided into l sections; and
S(yn): sectional area of ink flow path at position of yn.